The present invention relates broadly to a multiplexing system and method in a communications system wherein a plurality of low-volume users share the services of at least one high volume central unit or facility. More particularly, the present invention relates to a communication system and multiplexing system wherein the low-volume users are sychronously polled to ascertain whether or not the low-volume users desire access to the central facility and asynchronously polled to establish communication between the central facility and the low-volume user desiring access to the central facility.
Multiplexing is generally known in several prior systems to provide overall system economy of operation. Multiplexing systems include space division multiplexing, frequency division multiplexing, and time division multiplexing. Time division multiplexing is a commonly used technique in communications and computer/communications systems.
In time division multiplexing the common facility or central unit is used by no more than one user at any one time. The total available time of the common facility is segmented and each user is assigned a segment of time for use of the common facility. Systems for controlling usage of the common facility on the time shared basis include both synchronous and asynchronous control.
In synchronous control the time that the common shared facility has available is divided into time segments and each user is assigned a specific time segment. The users are then given access to the central unit in sequence and the sequence is repeated after the last user has been given access to the common facility. Time information must be provided to each user so that each user can ascertain when its time slot occurs. Each time slot is typically the same duration to minimize cost of communicating time information to the user. In the asynchronous control systems the time that the common facility is available is divided into segments that may be variable and dynamically changing in duration. The sequence of individual user access to the common facility may also be variable. Each user is informed as to when it may use the common facility immediately prior to being given access to the common facility. Once the remote user terminates its activity with the shared facility the system control selects the next user who will be provided access to the common facility.
In synchronous multiplexing control with each user assigned a specific time slot the user is free to use the central facility or not use the central facility during the assigned slot. In asynchronous multiplexing control, two basic modes are employed to determine the order of access to the common facility. In the contention system when two or more users simultaneously attempt to use the common facility the selection of the user is based on a priority system. In a polling system the remote users are individually queried on a periodic basis to ascertain whether or not each user desires to use the common facility. Polling may be implemented by centralized control and distributed control. In the centralized form of polling system all queries are initiated and transmitted from a single point to each individual user of the common facility in succession. In distributed form each user polls the next user in sequence once it has completed its use of the common facility.
While each type of multiplexing system, i.e. synchronous or asynchronous control, has certain advantages in specific circumstances, each also has significant disadvantages. Though the synchronous control method provides more efficient utilization of the common facility in environments where the duration of each use is constant, it also results in a user which does not need access to the common facility denying access to the user who does have need of the common facility. Similarly, even though asynchronous control provides more efficient utilization of the common facility in environments where the duration of use varies over a wide range of time and circumstances, it requires a polling contention resolving scheme to regulate access to the common facility.
In order to maximize the cost effectiveness of a communication system wherein a plurality of remote individual users share a high capacity central facility, it is desirable to alter the multiplexing control in accordance with the operating environment. One may do so by recognizing the two basic tasks performed in establishing communication between the central unit and a plurality of remote individual users. For the purposes herein, the word "terminal" is understood to mean a remote terminal location having at least one terminal station having a data source and/or a data sink. The first task is the identification of the remote user or terminal that has information available for transmission to the central facility. The other basic task is the communication of such information between the remote terminals and a central facility. The first task relates essentially to fixed length status messages lending itself to synchronous multiplexing control. The second task deals with text messages and information that may vary in size and duration, thus being amenable to asynchronous multiplexing control. The present invention is a hybrid multiplexing system and method that employs synchronous control for the status reporting task and asynchronous multiplexing control for all other operations including input and output of text information messages between central unit and the plurality of remote user terminals. Thus, the advantages of both synchronous and asynchronous polling is incorporated into the present invention optimizing the system performance. The present invention reduces the amount of time required to accomplish multiplexing and reduces the resources required to control the multiplexing process. By incorporating the hybrid multiplexing of the present invention the useful capacity of the common facility shared by the multiple users is increased.